1. Field of the Invention
The present invention relates to optical communication systems. More specifically, the present invention relates to wavelength converters for wavelength division multiplexing (WDM) applications.
2. Description of the Related Art
High-speed, high-capacity communication is currently enabled by the widespread use of optical communications technology. Optical modulators encode light of a predetermined wavelength with data. The encoded signal is then combined with other optically encoded signals and is transmitted over a medium. Typically, the medium is an optical fiber. Switching and routing of the optically encoded signals is effected using wavelength division multiplexers (WDMs) and optical switches.
Several approaches to wavelength conversion in optical WDM systems are known in the art. The simplest and most mature is an opto-electronic approach. With this approach, modulation carried by light at one wavelength is detected by an optical receiver and used to modulate light from another source at a different wavelength. That is, the first optical signal is converted to an electrical signal and modulated. The modulated signal is then converted from an electrical to an optical signal. In general, the all-optical techniques use nonlinear interactions between a signal and a probe beam to impart the signal""s modulation to light at a new wavelength. Thus, they require two optical inputs, the original signal being one of them.
An alternative approach involves the use of a saturable amplifier, such as a semiconductive device, as a gain medium. The gain medium is then modulated with an input optical signal. The modulated gain medium then is used to modulate a second beam having a second carrier wavelength relative to the input signal. Unfortunately, this approach also requires two lasers and a saturable amplifying gain medium. In addition the extinction ratio that can be achieved is quite small and it depends on the power of the input signal. Such wavelength converters can work only for amplitude-modulated signals. The change in the number of carriers in the SOA also leads to a change in refractive index and undesirable effects on the phase of the output. Therefore it is awkward and expensive. Another all-optical method uses four-wave mixing in an SOA of signal at frequency fs and probe at frequency fp producing modulated signal at frequency 2fpxe2x88x92fs]. Although this method is completely transparent to different bit rates and modulation formats, it has very low conversion efficiency, which rapidly decreases as the frequency separation increases.
Consequently, a need remains in the art for a more effective system or method for effecting wavelength conversion of an optical signal.
The need in the art is addressed by the frequency shifting device of the present invention. Generally, the inventive device includes a layer of optically refractive material having a moving refractive boundary responsive to an application of a traveling wave electrical signal and an arrangement for providing an electrical signal to the layer to effect a predetermined frequency shift of an optical signal passing therethrough.
In an illustrative embodiment, the device includes an active polymer layer, an optically conductive first cladding disposed above the active polymer layer, a microstrip line disposed over the first cladding layer, a second cladding layer disposed below the active polymer layer, and a ground plane disposed below the second cladding layer.
The present invention thereby provides a method for shifting a frequency of an optical signal comprising the steps of providing a layer of optically refractive material having a moving refractive boundary responsive to an application of an electrical signal and providing an electrical signal to the layer to effect a predetermined frequency shift of an optical signal passing therethrough.